1. Field of the Invention
The present invention relates generally to solid state imaging devices and, more particularly, to a new and improved acoustoelectric imager using a monolithic multistrip coupled convolver adapted to receive a projected image on a screen of radiation-sensitive semiconductor material.
2. Description of the Prior Art
The operation of acoustoelectric convolvers is based on the interaction of the normal components of the electric fields generated by two oppositely directed surface acoustic waves counter-propagating on a piezoelectric film or substrate with the charge carriers in a layer or wafer of semiconductor material. Earlier devices of this type employ a piezoelectric substrate of lithium niobate and a wafer of silicon spaced above the acoustic beam, or propagation path for the acoustic waves, on the substrate. The air gap spacing between the piezoelectric substrate and the silicon wafer is on the order of 1000 angstroms. For a discussion of such devices see, for example, G. S. Kino, W. R. Shreve, and H. R. Gautier, "Parametric Interactions of Rayleigh Waves," 1972 Ultrasonic Symposium Proceedings, IEEE Cat. No. 72 CHO 708-8SU, pages 285-287 and J. M. Smith, E. Stern, and Abraham Bers, "Accumulation-Layer Surface-Wave Convolver, " Electronics Letters, Vol. 9, No. 6, Mar. 22, 1973, pages 145-146. In the air-gap convolvers discussed in the above-referenced articles, the output signal is sensed across a pair of electrodes, one on the outer or top surface of the semiconductor wafer and one on the bottom or outer surface of the piezoelectric substrate.
In Gordon S. Kino and John Shaw, "Acoustic Surface Waves," Scientific American. Oct. 1972, pages 51-68, there is shown and discussed an air gap convolver operated as an imager wherein one line of an object is projected onto the semiconductor wafer through the piezoelectric substrate. The imager output is obtained by detecting one of the two counterpropagating surface acoustic waves at an interdigital transducer positioned to transduce the wave into an electrical signal after that wave has emerged from the interaction region between the semiconductor and the substrate. Scanning of the object in a direction orthogonal to the line is accomplished mechanically. The same device is discussed in more detail in N. J. Moll and C. F. Quate, "Scanning Optical Patterns with Acoustic Surface Waves," J. de Physique, Colloque C6, supplement to No. 11-12, Tome 33, page 231, Nov.-Dec. 1972.
A major disadvantage of air-gap convolvers is that a configuration which requires a semiconductor wafer to be precisely and uniformly spaced across an air gap from a piezoelectric substrate is difficult and expensive to manufacture in quantity to the tolerances required for consistent results. However, if the semiconductor wafer is placed in direct contact with the piezoelectric substrate, it interferes with the propagation of surface acoustic waves thereon. Recent interest has focused on the utilization of multistrip couplers which are arrays of parallel closely spaced narrow strips of conductor material disposed on the surface of a piezoelectric surface acoustic wave propagation medium perpendicular to the wave vectors. The multistrip coupler spans the acoustic beam and extends outward therefrom. Such a multistrip coupler is described in F. G. Marshall and E. G. S. Paige, "Novel Acoustic-Surface-Wave Directional Coupler with Diverse Applications," Electronics Letters, Vol. 7, No. 16, Aug. 12, 1971, pages 460-462. A multistrip coupler may be used to couple the normal components of electric fields associated with surface acoustic waves propagating on a piezoelectric film on a substrate to an adjacent region of the same substrate or to a similar distinct substrate. A strip of semiconductor material can then be placed in contact with the multistrip coupler outside the acoustic beam without interfering with the surface acoustic waves. A multistrip coupled convolver having a silicon chip with a thin silicon dioxide coating of controlled thickness mechanically pressed into contact with the coupling strips of the multistrip coupler is described in W. R. Shreve and G. S. kino, "Strip Coupled Acoustic Convolvers," 1973 Ultrasonics Symposium Proceedings, IEEE Cat. No. 73 CHO 807-8SU, pages 145-147.
While the mechanical contact model for a multistrip coupled convolver offers some advantages over air gap devices, the most promising possibility for the utilization of the multistrip coupled concept for convolvers is in its application to monolithic structures where the piezoelectric and semiconductor media are films deposited side by side on the same crystalline substrate. This approach has the advantage of enabling the fabrication of convolver devices to close tolerances in production quantities by the well established and relatively inexpensive techniques currently in use for the manufacture of integrated circuits.
Monolithic multistrip coupled convolvers are discussed by the inventor of the subject invention in L. R. Adkins, "Strip Coupled AlN and Si on Sapphire Convolvers," 1973 Ultrasonics Symposium Proceedings, IEEE Cat. No. 73 CHO 807-8SU, pages 148-151 and in L. R. Adkins, "Monolithic Aluminum Nitride/Silicon-on-Sapphire Strip Coupled Convolvers," 1974 Ultrasonics Symposium Proceedings, IEEE Cat. No. 74 CHO 896-1SU, pages 228-231.